The invention relates to the field of film filters sing omnidirectional reflectors.
Thin film reflectors have been widely used in many optical applications. See, for example, H. A. Macleod, xe2x80x9cThin-film optical filtersxe2x80x9d, second edition, (Adam Hiler Ltd, Bristol, England, 1986). Typically, such thin-film devices operate within a narrow range of angles. It has recently been discovered that a multi-layer film system can exhibit omni-directional reflectivity, i.e., light incident upon the film within a certain frequency range, with any angle of incidence, can be completely reflected. See U.S. Pat. No. 6,130,780 entitled xe2x80x9cHigh Omnidirectional Reflectorxe2x80x9d issued to Joannopoulos et al. Such technology allows the construction of thin-film reflector at much wider ranges of angles.
The invention provides a type of narrow band-pass filter. For a given wavelength, such a filter allows the transmission of light, or electromagnetic energy, at a single angle, or a small set of angles, while completely reflecting light from all other incidence angles. The invention also addresses the practical aspects regarding the use of such narrow band-pass filter for wavelength division multiplexing (WDM) applications.
The invention provides a type of thin film band-pass filter using omnidirectional reflectors. Such a filter transmits electromagnetic wave that is incident at a single incidence angle, or a small set of angles, while reflecting waves that are incident for all others angles. This filter could be important in order to perform channel add-drop functionality in wavelength division multiplexing applications.
Accordingly, the invention provides an electromagnetic wavelength filter that allows the transmission of electromagnetic energy within a narrow range of wavelengths while reflecting incident electromagnetic energy at other wavelengths. The filter includes at least one cavity region; and at least two reflectors surrounding the at least one cavity region, at least one of the reflectors being an omni-directional reflector. The omni-directional reflector includes a structure with a surface and an index of refraction variation perpendicular to the surface, and the omni-directional reflector is specifically configured to exhibit high omni-directional reflection for a predetermined range of frequencies of incident electromagnetic energy for any angle of incidence and any polarization.